Implementing Rotating Stall Control in a Radial Diffuser Using Microjet Arrays Jennifer Gavin, Erik Fernandez, Prabu Sellappan, Farrukh S. Alvi, William M. Bilbow and Sun Lin Xiang Abstract This study is part of our effort to implement and refine microjet-based flow control in realistic and challenging applications. Our goal is to reduce/eliminate rotating stall in the radial diffuser of a production compressor used in commercial heating, ventilation, and air conditioning (HVAC) systems, using microjet arrays. We systematically characterize the flow using pressure and velocity field measurements. At low load conditions, the flow is clearly stalled over a range of RPM where the presence of two rotating stall cells was documented. Circular microjet arrays were integrated in the diffuser and the flow response to actuation was examined. The array closest to the initiation of stall cells was most effective in reattaching the flow. Control led to a very significant increase in the stall margin, reducing the minimum operational mass flow rate to 14% of the design flow rate, half of the original 28% flow rate before microjet control was implemented. The results will show that the parameters found be most effective in the simple configurations proved to be near-optimal for the present surge control application in a much more complex geometry. This provides us confidence that the lessons learned from prior studies can be extended to more complex configurations. Keywords Active flow control · Microjets · Stall · Radial diffuser · PIV J. Gavin · E. Fernandez · P. Sellappan (B ) · F. S. Alvi Department of Mechanical Engineering, Florida Center for Advanced Aero-Propulsion (FCAAP), FAMU-FSU College of Engineering, Tallahassee, FL 32310, USA e-mail: psellappan@fsu.edu F. S. Alvi e-mail: falvi@fsu.edu W. M. Bilbow · S. L. Xiang Danfoss Turbocor, Tallahassee, FL 32310, USA © Springer Nature Switzerland AG 2019 R. King (ed.), Active Flow and Combustion Control 2018, Notes on Numerical Fluid Mechanics and Multidisciplinary Design 141, https://doi.org/10.1007/978-3-319-98177-2_3 33